An uninterruptible power supply unit is a power supply unit configured, when electric power is not supplied to a load device from an external power supply due to power failure, and the like, to supply electric power to the load device from a secondary battery charged beforehand, in order to continue the operation of the load device. Generally, the secondary battery of the uninterruptible power supply unit is charged by the external power supply at normal time. As an example of the secondary battery used for the uninterruptible power supply unit, an alkaline secondary battery, such as for example, a nickel-hydrogen secondary battery is known.
The alkaline secondary battery has a characteristic that it needs to be charged at a voltage higher than its rated voltage. However, generally, in the uninterruptible power supply unit, the rated voltage of the alkaline secondary battery is equal to the voltage of the external power supply. For this reason, when the voltage of the external power supply is used as it is, it is not possible to charge the alkaline secondary battery up to the fully charged state by electric power from the external power supply.
For example, when a secondary battery whose rated voltage is lower than the voltage of the external power supply is used, the secondary battery can be charged up to the fully charged state by electric power of the external power supply. Further, at the time of power failure, the load device can be operated by electric power of the secondary battery in such a manner that the voltage of the secondary battery is boosted up to the voltage of the external power supply by, for example, a DC to DC converter. However, in this case, the DC/DC converter, which boosts the voltage of the secondary battery at the time of power failure, needs to be provided, which causes problems of increase in size and cost of the uninterruptible power supply unit.
As a measure to cope with the problems, there is known a backup power supply unit configured such that the rated voltage of the secondary battery is set to be equal to the voltage of the external power supply, and such that, at the time of charging, the voltage of the external power supply is boosted by a DC/DC converter, and the secondary battery is charged with the output voltage of the DC/DC converter (see, for example, Patent Document 1). According to the conventional technique configured in this way, the DC/DC converter for boosting the voltage of the secondary battery at power failure is not needed, and thereby, the size and cost of the uninterruptible power supply unit can be reduced.